Process for preparing aerogels

ABSTRACT

The invention relates to a process for preparing modified SiO 2  gels, in which the gel is aged using a solution of a condensable alkyl and/or aryl orthosilicate, or using an aqueous silicic acid solution, in order to strengthen the structure of the gel.

DESCRIPTION

The invention relates to a process for preparing modified SiO₂ gels,referred to below as "aerogels".

Aerogels in the wider sense, i.e. in the sense of "gel with air asdispersion medium", are prepared by drying a suitable gel. The term"aerogel" in this sense includes aerogels in the narrower sense,xerogels and cryogels. A dried gel is referred to as an aerogel in thenarrower sense if the gel liquid is removed at temperatures above thecritical temperature, and starting at pressures above the criticalpressure. If the gel liquid is, on the other hand, removedsubcritically, for example with formation of a liquid-vapor boundaryphase, the resultant gel is referred to as a xerogel. It should be notedthat the gels prepared according to the invention are aerogels in thesense of gel with air as dispersion medium. Since these gels areprepared by subcritical drying, they may, however, also be referred toas xerogels.

SiO₂ aerogels are known for their excellent heat insulation efficiency.They can be prepared, for example, by acid hydrolysis of tetraethylorthosilicate in ethanol. The hydrolysis gives a gel whose structure canbe influenced by the temperature, the pH and the duration of the gellingprocess. However, the gel structure generally collapses when the wet gelis dried, since the capillary forces arising during the drying areextremely large. Collapse of the gel can be prevented by carrying outthe drying at a temperature above the critical temperature and at apressure above the critical pressure of the solvent. Since theliquid/gaseous phase boundary disappears in this region, capillaryforces are also absent, and the gel does not change its nature duringthe drying process, i.e. there is also no shrinkage of the gel duringdrying. Preparation processes based on this drying technique are known,for example, from EP-B-0 396 076 and WO 92/03378. This techniquerequires, however, for example if ethanol is used, a temperature greaterthan 240° C. and pressures of up to 40 bar.

Although the drying temperature is reduced to about 40° C. if ethanol isreplaced by CO₂ before drying, the necessary pressure, however, is then80 bar.

WO 92/20623 discloses a process in which SiO₂ aerogels are prepared byhydrolysis and polycondensation of tetraalkoxysilanes, wherein theaqueous/alcoholic gel is aged in a solution of tetraalkoxysilanes inorder to increase the strength of the gel structure. However, the priorart has the disadvantages that extremely expensive raw materials, suchas tetraalkoxysilanes, are employed both for preparing and for aging ofthe gels, and that the dwell times necessary for aging of the gel andstrengthening of the structure are very long.

It has now been found that deionized SiO₂ gels, prepared frominexpensive water glass, can be dried under sub-critical conditions if,before drying, they are aged in an aqueous/organic solution of alkyland/or aryl ortho-silicate and/or free silicic acid. The productsobtained are referred to below as "aerogels", and have outstanding heatinsulation efficiency.

The invention therefore relates to a process for preparing aerogels,which comprises

a) bringing an aqueous water glass solution to a pH of ≦3, using an acidion exchanger or a mineral acid,

b) polycondensing the silicic acid thus produced, by addition of a base,to give an SiO₂ gel, and, if a mineral acid was used in step a), washingthe gel with water until it is free of electrolytes,

c) if required, washing the gel obtained in step b) with a suitablealcohol or organic solvent until the water content of the gel is <20% byweight,

d) aging the gel obtained in step b) or c), using a solution of acondensable alkyl and/or aryl ortho-silicate of the formula R¹ _(4-n)Si(OR²)_(n), where n is from 1 to 4 and R¹ and R², independently of oneanother, are C₁ -C₆ -alkyl, cyclohexyl or phenyl, or using an aqueoussilicic acid solution, in order to strengthen the gel structure,

e) subcritically drying the aged gel obtained in step d).

In step a), an acid ion-exchanger resin is preferably employed, thosecontaining sulfonic acid groups being particularly suitable. If mineralacids are employed, hydrochloric and sulfuric acid are especiallysuitable. The water glass used is in general sodium water glass and/orpotassium water glass.

The base used in step b) is preferably NH₄ OH, NaOH, KOH, Al(OH)₃,colloidal silicic acid and/or water glass. If a mineral acid is used instep a), the SiO₂ gel which is formed by the action of the base iswashed with water until it is free of electrolytes; the washing ispreferably continued until the washings have the same electricalconductivity as deionized water.

Before step c), the gel is preferably aged, in general at a temperaturebetween the freezing point and the boiling point of the solution,preferably at from 0° to 120° C., particularly preferably at from 60° to100° C., and at a pH of from 4 to 11, preferably from 4 to 9. Theduration of the aging is generally from 10 seconds to 48 hours,preferably from 10 seconds to 5 hours.

If the gel strengthening in step d) is carried out using a solution ofan alkyl and/or aryl orthosilicate, the gel is preferably washed in stepc) with a suitable alcohol or organic solvent until the water content is<10% by weight. The alcohols generally used are linear or branchedaliphatic alcohols, preferably methanol, ethanol, propanol, isopropanol,butanol or isobutanol. It is also possible to employ other organicsolvents which are miscible with water, for example THF or acetone, ormixtures of such solvents.

If the gel strengthening in step d) is carried out using an inorganic,low-molecular-weight silicic acid, the solvent in step c) is preferablywater. It may be advantageous to perform a solvent exchange byintroducing organic admixtures (alcohols, aldehydes and/or ketones) intothe aqueous phase, and to vary the pH between 3 and 11 in order toinfluence the rate of condensation and the deposition of the SiO₂ fromthe silicic acid which is added in step d).

The gel strengthening in step d) is carried out by introduction (eg.diffusion) of a condensable SiO₂ source into the pore space of the gelprepared in step b) and, if required, treated in step c), and thendepositing the SiO₂ source on the existing gel framework by acondensation reaction.

The SiO₂ source employed can be either an alkyl and/or arylorthosilicate of the formula R¹ _(4-n) Si(OR²)_(n) or a dilute solutionof a low-molecular-weight silicic acid. If the gel strengthening is tobe carried out using the abovementioned orthosilicate or mixtures ofdifferent orthosilicates of the same formula, then the gel treated instep c) is aged in an alcoholic solution of the orthosilicate. Theorthosilicates employed are preferably orthosilicates of the formula R¹_(4-n) Si(OR²)_(n) where n is from 1 to 4, and R¹ and R², independentlyof one another, are C₁ -C₆ -alkyl, cyclohexyl or phenyl. Tetraethyland/or tetramethyl orthosilicate is/are particularly preferable. Theconcentration of the orthosilicate in the alcoholic solution is from 0.1to 30% by volume, preferably from 1 to 10% by volume. The alcohols usedare generally linear or branched aliphatic alcohols, preferablymethanol, ethanol, propanol, isopropanol, butanol or isobutanol. Theduration of the aging is in general from 10 minutes to 48 hours,preferably from 10 minutes to 24 hours.

As an alternative to the use of orthosilicates in step d) for gelstrengthening, dilute aqueous solutions of low-molecular-weight silicicacid and/or of alkali metal silicates can also be used. Use of freesilicic acid, obtained, for example, from an aqueous solution with thehelp of an abovementioned ion exchanger, has the advantage that none ofthe salt ions which derive from the pH adjustment and neutralization ofthe water glass solution and adversely affect the drying process remainin the gel. Preference is given to a silicic acid solution having aconcentration of from 1 to 10% by weight, particularly preferably from 4to 7% by weight, prepared, for example, by cation exchange on anabovementioned ion-exchange resin. Organic fractions, such as ketones,aldehydes or branched or unbranched alcohols may, furthermore, be addedto the solution in suitable concentration in order to influence thepolycondensation of the silicic acid, the growth of clusters in thepolycondensates during their formation, and the deposition on the gelframework formed in step b) and, if required, treated in step c). Theupper limit of the total organic admixture is determined by theprecipitation of the first SiO₂ fractions.

Steps a) to d) are preferably carried out at a temperature between thefreezing point of the solution and 150° C., and at a pressure of from 1to 10 bar.

In step e), the aged gel is dried subcritically, preferably attemperatures from -30° to 200° C., particularly preferably from 0° to150° C. The pressures applied during drying are preferably from 0.001 to20 bar, particularly preferably from 0.01 to 5 bar. The drying isgenerally continued until the gel has a residual solvent content of lessthan 0.1% by weight.

The novel process is described in greater detail below by means of aworking example.

EXAMPLE

1 l of a sodium water glass solution (containing 8% by weight SiO₂ andwith a Na₂ O:SiO₂ ratio of 1:3.3) is passed (20 ml/min) through a column(diameter: 50 mm, length: 300 mm) filled with 0.5 l of an acidion-exchange is resin (styrene-divinylbenzene copolymer with sulfonicacid groups, commercially available as Duolite® C20). The pH of theeluted solution is 2.3. The solution is then set to a pH of 5.6 using 1molar NaOH solution, after which the resultant gel is mechanicallycomminuted with a stirrer (mean particle size <0.5 mm) and the water isthen separated from the gel using ethanol at 50° C. in acontinuously-operating extraction vessel to a residual water content inthe gel of <10% by weight. The gel is then aged at 60° C. for 24 hoursin an ethanolic solution of tetraethyl orthosilicate (concentration 10%by volume). The drying of the gel is carried out in a nitrogen-flusheddryer (6 hours at 60° C. and 12 hours at 150° C.).

The transparent aerogel thus obtained has a density of 0.25 g/cm³, a BETspecific surface area of approximately 820 m² /g and a λ value of 0.027W/mK.

The thermal conductivity was measured by a hot wire method (see, forexample, O. Nielsson, G. Ruschenpohler, J. Grob, J. Fricke, HighTemperatures--High Pressures, Vol. 21, 267-274 (1989)).

We claim:
 1. A process for preparing aerogels, which comprisesa)bringing an aqueous water glass solution to a pH of ≦3 using an acid ionexchanger or a mineral acid, b) polycondensing the silicic acid thusproduced, by addition of a base, to give an SiO₂ gel, and, if a mineralacid was used in step a), washing the gel with water until it is free ofelectrolytes, c) if required, washing the gel obtained in step b) with asuitable alcohol or organic solvent until the water content of the gelis <20% by weight, d) aging the gel obtained in step b) or c), using asolution of a condensable alkyl and/or aryl ortho-silicate of theformula R¹ _(4-n) Si(OR²)_(n), where n is 3 or 4 and R¹ and R²,independently of one another, are C₁ -C₆ -alkyl, cyclohexyl or phenyl,or using an aqueous silicic acid solution, in order to strengthen thegel structure, e) subcritically drying the aged gel obtained in step d).2. The process as claimed in claim 1, wherein the base used in step b)is NH₄ OH, NaOH, KOH, Al(OH)₃, colloidal silicic acid and/or waterglass.
 3. The process as claimed in claim 1, wherein the SiO₂ gelobtained in step b) is aged at a temperature between the freezing pointand the boiling point of the solution, and at a pH of from 4 to 11, overa period of from 10 seconds to 48 hours, before the water is removed instep c).
 4. The process as claimed in claim 1, wherein linear orbranched aliphatic alcohols are used in step c).
 5. The process asclaimed in claim 1, wherein the alcohol used in steps c) and/or d),independently of one another, is methanol, ethanol, propanol,isopropanol, butanol or isobutanol.
 6. The process as claimed in claim1, wherein, in step d), tetramethyl orthosilicate and/or tetraethylorthosilicate is/are used.
 7. The process as claimed in claim 1,wherein, in step d), the concentration of the orthosilicate in thealcoholic solution is from 0.1 to 30% by volume.
 8. The process asclaimed in claim 1, wherein, in step d), the gel is aged for from 10minutes to 48 hours.
 9. The process as claimed in claim 1, wherein stepsa) to d) are carried out at a temperature between the freezing point ofthe solution and 150° C. and at a pressure of from 1 to 10 bar.
 10. Theprocess as claimed in claim 1, wherein, in step e), the aged gel isdried at from -30° to 200° C. and at from 0.001 to 20 bar.